1. Field of the Invention
The present invention relates to an apparatus for controlling ignition timing in an internal combustion engine.
2. Description of the Prior Art
A typical conventional apparatus for controlling ignition timing in an internal combustion engine is disclosed, e.g., in official gazettes of Japanese Patent Disclosure No. 59060/1982 and Japanese Patent Disclosure No. 59061/1982. FIG. 8 is a sectional view of an internal combustion engine for which the conventional apparatus is employed wherein the whole structure of the apparatus is schematically illustrated in the drawing. In the drawing, reference numeral 1 designates an air cleaner, reference numeral 2 designates an air flow meter for measuring a quantity of intake air, reference numeral 3 designates a throttle valve, reference numeral 4 designates an intake air manifold, reference numeral 5 designates a cylinder, reference numeral 6 designates a coolant temperature sensor for detecting the temperature of a coolant flowing through the engine, reference numeral 7 designates a crank angle sensor, reference numeral 8 designates an exhaust gas manifold, reference numeral 9 designates an exhaust gas sensor, reference numeral 10 designates an injector, reference numeral 11 designates an ignition plug, reference numeral 12 designates a controller and reference numeral 13 designates an igniter.
With the conventional apparatus as constructed in the above-described manner, the crank angle sensor 7 outputs to the controller 12 a reference position indicative pulse per each reference position for a crank angle of the crankshaft (e.g., per 180.degree. in the case of a four-cylinder type internal combustion engine; per 120.degree. in the case of a six-cylinder type internal combustion engine). In addition, the crank angle sensor 7 outputs to the controller 12 a unit pulse per unit crank angle of the crankshaft. After the controller 12 has the reference position indicative pulse inputted thereinto, it calculates the number of inputted unit angle indicative pulses to determine the present crank angle of the crankshaft. Further, the controller 12 can determine the present engine speed by measuring a frequency or a period of the unit angle pulse. Incidentally, the crank angle sensor 7 is normally mounted in a distributor.
The controller 12 is normally constituted in the form of a microcomputer including, e.g., CPU, RAM, ROM, an input/output interface and so forth. An intake air quantity indicative signal S1 transmitted from the air flow meter 2, a coolant temperature indicative signal S2 transmitted from the coolant temperature sensor 6, a crank angle indicative signal S3 transmitted from the crank angle sensor 7, an air/fuel ratio indicative signal S4 transmitted from the exhaust gas sensor 9 and a signal (not shown) indicative of a fully closed state of the throttle valve 3 are inputted into the controller 12, respectively. In response to these signals, the controller 12 performs calculations to calculate an ignition timing and a quantity of fuel to be injected. Subsequently, the controller 12 outputs an ignition signal to the igniter 13 such that ignition occurs at the calculated ignition timing. After the ignition plug 11 is activated, the controller 12 outputs a driving signal S5 to drive the injector 10 with a predetermined air/fuel ratio.
To correctly calculate the ignition timing, a basic ignition timing corresponding to an engine speed N and an intake air quantity Q is previously stored in the controller 12. When the engine starts, the controller 12 reads the engine speed N and the intake air quantity Q and then determines a quantity of correction to be made by performing addition or multiplication in consideration of the temperature of a coolant and other factors, whereby a desirable ignition timing is obtained.
With the conventional apparatus as described above, however, it has been found that the apparatus has a problem that the controller cannot carry out ignition timing control at which a maximum torque is obtainable, if a point representative of minimum advance for best torque (hereinafter referred to as an MBT point) at the time when an ignition timing map is set is different from an MBT point at the time of practical usage, because engine characteristics vary as time elapses and a certain amount of productive fluctuation unavoidably occurs with the apparatus.